Improve `indexing_slicing` help message

[shnewto]

The help message reported by the indexing_slicing lint could be improved to be more specific. Currently its reports are relatively generic suggestions, i.e. slicing may panic. Consider using .get(..n) or .get_mut(..n) instead.

It would be useful if we figured out whether get() or get_mut() were the right function to suggest and reported the index/span's actual value. After these changes the above example might look something like slicing may panic. Consider using .get_mut(..3) instead